It is known from German open application DE-OS 32 16 886, for example, to fabricate gypsum/fiber board in the aforedescribed manner utilizing so-called plaster of Paris as the gypsum binder, i.e. calcium sulfate beta hemihydrate. The prepared mixture is spread onto a pressing plate and is shaped to the intermediate product which has a mat-like character.
The pressing is effected between the press plates of a platen press with the periphery of the board being open in the sense that the board is pressed between a pair of spacer bars so that the sides of the board extending between the bars are not flanked by board-shaping members.
The gypsum binder and the fibers forming the dry mix are agitated during the supply of the preparation water. The preparation water is sprayed into the agitated dry mix in finely-divided form. The prepared mixture, before setting, is spread onto a synthetic resin web and then pressed in the aforedescribed manner. The press pressure is maintained for a length of time sufficient to impart to the intermediate board a sufficient strength.
The thickness of the gypsum/fiber board is determined primarily by the so-called spacer bars which are employed by placing them upon the lower press plate and which flank only two sides of the intermediate board while the two other sides remain peripherally open. As a consequence, the pressure upon the intermediate board tends to decrease toward these edges
As a practical matter, it has been found to be necessary to subject the product to a subsequent trimming. If spacer bars are used but the spreading of the prepared mass is not sufficiently uniform, during the pressing of the intermediate product to the finished gypsum/fiber board local pressure differences arise in the pressed product and the gypsum/fiber board will not have sufficiently homogeneous physical parameters such as density and strength.
In addition, the gypsum/fiber board is removed from the press in the conventional system in the form of relatively limp boards or sheets, i.e. so-called green board, which must be supported for further handling and processing operations since the green board is not sufficiently self-supporting.
All of these disadvantages are found in the earlier system which also provides a relatively high preparation of fibers in the prepared mass and requires a relatively large amount of preparation water. As a consequence, water must be expressed from the mass and the pressed-out water must be carried away.
In practice, it has been found further that when the gypsum binder is plaster of Paris, i.e. calcium sulfate beta hemihydrate, and the fibers are cellulose-containing fibers, it is practically impossible to prevent relatively large amounts of air from being entrained into the product in the prepared mixture, so that, during pressing, the entrained air is only partly expelled As a consequence, the plaster board which is obtained by this method shows significant density variations.
The thicker the intermediate product and the pressed gypsum/fiber board, the more these drawbacks affect the quality of the finished product. Indeed it is possible to provide uniform gypsum/fiber board which is of relatively small thickness. Highly stressed gypsum building board, especially floor board, however, of a relatively thick nature could not satisfactorily be made heretofore except by cementing together a plurality of relatively thin gypsum/fiber boards, and indeed while ensuring crossing orientations of the fibers of the boards which are oriented during the spreading operation. The cementing together of thin gypsum/fiber board is expensive.